The invention relates to a reversing transmission shiftable under load which preferably has a hydrodynamic torque converter, an input and an output shaft, at least one countershaft and gear wheels; which form a reversing set having shifting clutches with idler wheels which have actuating devices for changing direction which can, optionally, be non-rotatably connected with the input shaft or countershaft.
The transmission, the kind described above, is a reversing transmission, such specially as used in industrial lift trucks, e.g. loaders. It also can be the input unit of a multi-gear reversing transmission, such as specially used in construction machinery, e.g. wheel loaders. Reversing transmissions shiftable under load have several advantages. These transmissions must be compactly built and comfortable to shift. In addition, it is important that the hydraulically actuatable shifting clutches are shifted by modulation of the actuating pressure in such a manner as to have a controlled load take-up. In certain vehicles, e.g. in industrial lift trucks such as forklift trucks or the like, together with the reversing operation, other operating modes are important. Among these are mainly starting, inching and braking.
In DE 195 17 888 is disclosed a reversing transmission shiftable under load having hydrodynamic converter in which the shifting clutches are designed as negative clutches. The modulation of the shifting clutches during a reversal shift is obtained by means of the spring accumulator and the amount of oil flowing out via an orifice. It must be certain that exclusively pressure means and no air be in the piston chambers of the shifting clutches and in the lines, as this disadvantageously affects the modulation of the shifting elements, since in the case of pressure breakdown the air is released thereby changing the shifting time. Since a certain amount of air can get into the pressure means when operating the drive train by rotation of the gears, it is not possible in the transmission disclosed to prevent that pressure fluid from mixing with air sucked by the pump and reaching the lines and piston chambers.
The problem on which this invention is based is to provide a reversing transmission in which the piston chambers and the feed lines thereof are air-free and filled with pressure medium which remains air free during the operation and int the initial starting operation of the drive train and can evacuate themselves and fill with pressure medium.
According to the invention, the problem is solved by the features stated in the characteristic part of claim 1. The piston chamber has a first bore for supply of pressure medium which is preferably placed on the highest point of the piston chamber and through which the piston chamber, with the aid of an actuating device, can communicate either with the pressure means conveyed by a pump or via an orifice with an oil reservoir. By placing a second intake bore in the piston chamber, which is spatially located beneath the first intake bore and is permanently connected with the rate of flow of the pump but at a lower pressure level, a pressure mechanism always flows through the piston chamber when the upper intake bore is connected with the oil reservoir via the actuating device. Thereby air existing in the piston chamber is driven by the pressure medium flowing in through the lower intake bore out of the piston chamber into the upper bore through which it can flow out via the orifice to the oil reservoir. A non-return valve in the lower intake bore prevents the pressure medium from flowing out into the lower intake bore when the upper intake bore is connected with the pump via the actuating device in order to open the clutch. An orifice in the lower intake bore is designed so as to prevent the amount of the pressure medium flowing in through the lower intake bore being larger than the amount flowing out through the upper intake bore via an orifice into the oil reservoir. Undesired pressure build up in the piston chamber is thereby prevented. The reversing transmission shiftable under load has at least two reversing clutches of which one optional clutch is always connected with the pump via the actuating device. Thereby one clutch is in an open state and the non-return valve in the lower intake line of this clutch is closed by the pressure in the piston chamber. A second clutch is connected with the oil reservoir via the actuating device by way of an orifice and is in closed state, its piston chamber being permanently traversed with pressure fluid from the lower intake bore through the orifice in the piston chamber and from there, via the upper intake bore, into the oil reservoir. In a reverse shifting by changing over the actuating device, pressure fluid flows through the first clutch and thereby evacuates it while it is in closed state and the second clutch is opened by pressure fluid. While in initial starting, by repeated operation of the actuating device pressure fluid optionally flows through the first or the second reversing clutch, in each reversing clutch, the air present in the piston chamber is diverted to the oil reservoir. Thereby, even without additional evacuation operations, it is ensured that the intake lines and piston chambers become and remain airless. It is also possible to place the first bore for pressure medium supply beneath the second bore through which pressure fluid always flows through the piston chamber when the first bore is not subjected to pressure. In this case, it must be ensured that the current fluid flow be large enough to entrain and carry off the air present in the piston chamber. In a reversing clutch in which the clutch is closed by spring tension and re-opened by applying a hydraulic pressure and in which it is ensured that there be no air in the intake lines and in the piston chambers, it is always possible to carry out a uniform pressure modulation during the reverse shifting. For this purpose, in the non-return line to the oil reservoir, one orifice is provided with which a spring accumulator is connected in parallel. The accumulator used is designed for reverse shifting. This means that it almost fully absorbs the oil removed in sudden bursts from the piston chamber when actuating the clutch. The subsequent modulation operation develops automatically so that the driver is not burdened from the pressure modulation. The time function to close a shifting clutch can be adjusted by simply varying the orifice opening.